HAL Id: jpa-00220939
https://hal.archives-ouvertes.fr/jpa-00220939
Submitted on 1 Jan 1981
HAL is a multi-disciplinary open access archive for the deposit and dissemination of sci- entific research documents, whether they are pub- lished or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers.
L’archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d’enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.
COMPARISON OF OPTICALLY INDUCED
LOCALIZED STATES IN CHALCOGENIDE GLASSES AND THEIR CRYSTALLINE COUNTERPARTS
S. Bishop, B. Shanabrook, U. Strom, P. Taylor
To cite this version:
S. Bishop, B. Shanabrook, U. Strom, P. Taylor. COMPARISON OF OPTICALLY INDUCED LO- CALIZED STATES IN CHALCOGENIDE GLASSES AND THEIR CRYSTALLINE COUNTER- PARTS. Journal de Physique Colloques, 1981, 42 (C4), pp.C4-383-C4-386. �10.1051/jphyscol:1981482�.
�jpa-00220939�
COMPARISON OF O P T I C A L L Y INDUCED L O C A L I Z E D S T A T E S I N CHALCOGENIDE GLASSES AND T H E I R C R Y S T A L L I N E COUNTERPARTS
S.G. Bishop, B.V. Shanabrook, U. Strom and P.C. Taylor Naval Research Laboratory, Washington, D. C. 20375, U.S. A.
Abstract.- The below-gap peaks observed in the PLE spectra for deep PL bands in the minerals orpiment and realgar are attributed to strong impurity absorp- tion bands, possibly associated with the high concentrations of 3d transition metals (e.g. Mn and Fe) detected by ESR. It is suggested that the weak below- gap tails in the PLE spectra of chalcogenide glasses and synthetic arsenic chalcogenide crystals have the same origin, but that impurity concentrations are much lower in these materials. Optically induced ESR ~pectra~qbserved in crystalline arsenic chalcogenides are ascribed to paramagnetic Cu ions rather than to the optically induced paramagnetic intrinsic defects associated with major constituent atoms as observed in chalcogenide glasses.
Introduction.- Previous studies (1) have demonstrated that the luminescence spectra obtained in crystalline AsZSe3 and As2S3 are quite similar to those of the corre- sponding glasses. This observation has led to the inference that the same recombi- nation mechanisms or centers are present in both crystalline and amorphous chal- cogenides. Subsequent to this early work, luminescence (2) and a variety of opti- cally induced effects (3) have been studied extensively in chalcogenide glasses, but comparatively few studies of luminescence in crystalline chalcogenides have been reported (4-7). In the present work photoluminescence (PL), photoluminescence excitation (PLE) spectroscopy, PL fatigue, electron spin resonance (ESR), and opti- cally induced ESR studies have been carried out in the minerals orpiment (As S ) and realgar (As4S4). On the basis of the results it is suggested that impurities can 2 3 play an important role in determining the optically induced properties of crystal- line arsenic chalcogenides.
Results and Discussion.- The photoluminescence and ESR techniques and apparatus employed in these experiments have been described in detail in ref. (3). Typical PL and PLE spectra obtained at 4.2K from two different samples of orpiment and a sample of realgar are shown in Fig. 1. In all cases two broad PL bands were observed at energies well below the band edge. The deeper PL band observed in the orpiment crystals in the vicinity of 1.1-1.2 eV is similar to the bands reported previously in orpiment (1,4,5) and described as corresponding to the PL band observed in the glass. The energies of both the PL bands vary from crystal to crystal, consistent with the varying energies which have been reported previously for the PL bands in orpiment. This variation from crystal to crystal suggests that the PL bands might be associated with impurity centers rather than intrinsic defects or that impurities might influence the properties of the intrinsic defects. The most likely explanation for the sharply structured band with zero phonon lines (Fig. l(b)) observed in one of the crystals with 476& excitation is that it involves intracenter d-d transitions on a 3d transition metal impurity ion. This is consistent with the ESR spectra described below which demonstrate that substantial concentrations of transition metal impurities, as well as possible polycrystalline oxide inclusions, are present in these naturally occurring crystals.
In all cases the luminescence exhibits a large Stokes shift. That is, the PL bands are centered -1 eV below the peak energies of their corresponding PLE spectra.
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1981482
C4-384 JOURNAL DE PHYSIQUE
-
UJk
Z
3 Fig. 1. The PL (0.9-1.9 eV) and PLE (1.8-
> 3 eV) spectra of various arsenic sulfide
(r a crystals, (a) orpiment (Mexico), (b) orpiment
0: t-
-
(Turkey) and (c) realgar (Nevada). The PLm
(r spectra were obtained with 52082 (solid) and
a
-
47622 (dotted) radiation. The PLE spectra> of portions of the PL band between 1.5-1.8 eV
k (dotted) and 1.1-1.3 eV (solid) are also
UJ z shown.
W t- Z
10 1 8 2 6
ENERGY (EV)
These large Stokes shifts and the broad PL bands (fwhm
-
0.2-0.4 eV) are character- istics which strongly resemble those of the PL spectra observed in chalcogenide glasses, as are the strongly peaked PLE spectra (2). However, in the case of orpi- ment, the peaks in the PLE spectra occur well below the optical absorption edge (8) in the spectral range where the weak below-gap tails in the PLE spectra of synthetic chalcogenide crystals and chalcogenide glasses have been observed (5,7). (In chal- cog~nide glasses the peak in the PLE spectrum occurs nearly universally at G I 0 0 cm .) Street et al. (7) have shown that the PLE spectra of As S glass and syn- thetic crystals of 85As Se :15As S ex ibit weak below gap taiZs from which ab- 3borption coefficients as210$ as 0 2 1 3 c ~ 9 were derived. These workers attributed the low energy bulk absorption tails to transitions between negatively charged PL centers near the valence band edge and the opposite conduction band edge. After this exci- tation a strong local lattice distortion occurs (2,7).
In the high purity synthetic chalcogenide crystals (5) and in the pure chal- cogenide glasses (7) the excitation of.luminescence via the below gap absorption tails is weak in comparison to the above gap excitation. However, in the natural crystals we have studied, excitation in the below gap spectral range provides the dominant feature of the PLE spectra. Previous studies of the optical absorption tails in Fe doped As2S3 (9,lO) and in specially distilled chalcogenide glasses (11) have demonstrated that impurities can contribute strongly to the absorption in this below gap spectral range. We suggest that the below gap peaks in the PLE spectra of
"g. 1 correspond to impurity induced extrinsic absorption bands which efficiently
excite the strongly Stokes shifted PL bands. Such absorption bands can be expected
LO be much stronger in the highly impure minerals than in the high purity glasses or synthetic crystals. Thus PLE in this spectral range is dominant for the minerals.
The PL and PLE spectra for orpiment in Fig. 1 conform to the classic description (2,12) of luminescence and absorption in a system with strong phonon coupling for the case where the excited state is discre e. In this case the PL linewidth 0 is related to the Stokes shift 2W by a=(2YM))', where Q is the dominant phonon frequency.
Using the experimental Stokes shifts, as seen in Fig. 1, and the dominant phonon energy for As S (-0.03 eV), the above equation predicts linewidths 0 -0.2 eV, which are in reasonibqe agreement with observation.
it can be inferred that the PLE peak for the 0.9 eV PL band occurs below the absorp- tion edge. However, both the position in energy and the shape of the higher energy PLE spectrum (i.e. steep onset at the band gap energy and a long tail extending to higher energy) indicate that it is probably attributable to interband absorption
(valence band to conduction band excitation).
Ali of the PL bands shown in Fig. 1 exhibit significant fatigue during con- tinuous excitation by laser radiation. This includes the sharply structured band observed in the orpiment crystal from Turkey; the physical origin of this band obviously differs from those of the other broad, featureless bands. Since the various unrelated radiation processes all fatigue at comparable rates we suggest that the fatigue effect may be attributable to the introduction of an optically induced, independent, competing non-radiative process or center as has been previously suggested for glassy Se (13). It seems unlikely that the parallel fatigue rates involve optically induced changes in the radiative recombination centers themselves which render them non-radiative. Previous studies (2,3) of optically induced effects have suggested a possible relationship between the PL fatigue process and the observation of optically induced ESR. For this reason we have attempted to observe optically induced ESR in the same naturally occurring crystals of As S as used in the PL experiments.
2 3
Parallel electron spin resonance (ESR) measurements of orpiment indicate the presfgce-gf transition metal impurities, often in significant concentrations
(210 cm ). All samples contained iron either as antiferromagnetic ferromagnetic Fe-rich precipitates (probably inl&he- o m of an iron qlide) or as Fegf. Some
samples contained measurable (-10 cm
')
amounts of Cu 2$n an octahedral environment.Even when single crystals of As S were employed, the Cu spectrum occurred as a powder average (i.e., independent of the orientation of the crystal with respect to 2 3 the magnetic field) which suggests that the Cu is either interstitial, intercolated between the layers or precipitahsd in f8poiycrystalline phase such as an oxide. One sample (from Peru) contained Mn (210 cm ) which also exhibited an orientationally independent ESR spectrum. Sillall crystal field splittiags indicate that the manganese in As S exists in a fairly symmetric environment. In addition to these species several as yet unidentified features were observed which we believe are also the 2 3 result of transition metal impurities.
Several samples of
orpisnnt
were irradiated at 77K with 52082 (Kr laser) at a power density of5
1 watt cm.
The resulting ESR spectrum at 4.2K is shown for one sample in Fig. 2 (middle trace) along with the response before irradiation (top trace). Similar results were obtained on other samples. The optically .(or X-ray) induced ESR signal can be thermally annealed by cycling to 300K for several minutes;no attempt was made to bleach the signal optically. The optically indy$ed ESR response in naturally occurring orpiment is almost certainly dueZfo Cu and not to any intrinsic defect of the AsZS3 structure. The spectrum of Cu observed in a film of glassy As2Se3, which was evaporated onto a room temperature substrate (141, is shown in Fig. (bottom) for comparison. Although the Cu hyperfine structure on the low field side of the line is not as well defined on the As S trace, the general lineshaps including the low field asymmetry, is virtually2iaeotical to that attributed to Cu in glassy As2Se3. In contrast, the optically induced line attributed (3) to a hole localized on a sulfur atom in glassy As2S3 is 99th more symmetric than the response of Fig. 2 and narrower (706 vs. llOG for Cu 1.
The behavior just described for orpiment is similar to that which 99s been observed for good, vapor grown cryssgls of As Se where paramagnetic Cu is induced
2 3
after x-irradiation at 77K. The Cu response in crystalline As2Se3 can also be thermally annealed by cycling to 300K, although the annealing is not as complete as it is in As S
2 3'
C4-386 JOURNAL DE PHYSIQUE
1 . I
2500 MOO 3500 4000 MAGNETIC FIELD (G) z
P
43
8
w
There is thus no evidence in either crystalline As2S3 or in undoped, crystal- line As2Se3 for intrinsic, optic@ly-induced paramagnetxc defects, although both crystals exhibit paramagnetic Cu after x-irradiation. Since optically induced changes in the charge state of transition metal dopants in crystalline semicon- ductors have been known to quench luminescence, such optically induced charge transfer processes might give rise to the PL fatigue observed in the crystalline arsenic chalcogenides.
Fig. 2. ESR spectra of orpiment (Mexico) at 4.2K and 9.098 GHz before irradiation (to ) and after several minutes irradiation at 52088 a 77K (middle). The bottom trace is due to Cu
h+
in a glassy As2Se3 film.Acknowledgements.- The authors with to thank Dr. Richard Zallen for providing a series of orpiment crystals and for helpful discussions. Dr. W. M. Pontuschka is gratefully acknowledged for help with the initial ESR measurements.
References
KOLOMIETS B.T., MAMONTOVA T.N. and BABAEV A.A., J. Non-Crystl. Solids
4
(1970)289.
STREET R.A., Adv. in Physics
25
(1976).BISHOP S.G., STROM U. and TAYLOR P.C., Phys. Rev. B
15
(1977) 2278.MOLLOT F., CERNOGORA 3. and BENOIT 3 la GUILLAUME C., Phys. Stat. Solidi (a)
21
(1974) 281.
STREET R.A., AUSTIN I.G., SEARLE T.M. and SMITH B.A., J. Phys. C: Solid State Phys.
1
(1974) 4185.STREET R.A., AUSTIN I.G. and SEARLE T.M., J. Phys. C: Solid State Phys.
I!
(1975) 1293).
STREET R.A., SEARLE T.M. and AUSTIN I.G., Philos. Mag.
2
(1975) 431.ZALLEN R., DREWS R.E., EMERALD R.L. and SLADE M.L., Phys. Rev. Lett.
26
(1971) 1564.
TAUC J., DI SALVO F.J., PETERSON G.E. and WOOD D.L., Amorphous Magnetism, ed. H.O. Hooper and A.M. de Graaf (New York, Plenum Press, 1973), p. 119.
BISHOP S.G. and TAYLOR P.C., Philos. Mag. B
3
(1979) 483.HILTON A.R., HAYES D.J. and RECHTIN M.D., Chalcogenide Glasses for High Energy Laser Applications, Report No. 08-74-44 (Texas Instruments).
12. KLICK C.C. ~ ~ ~ - S C H U L M A N J.H. ,-solid State Physics Vol. 5, ed. F. Seitz and D.
Turnbull (Academic Press, New York, 1957), p. 97.
13. BISHOP S.G. and TAYLOR P.C., J. Non-Crystl. Solids
35-36
(1980) 909.14. TAYLOR P. C., FRIEBELE E. J. and BISHOP S. G., in Physics of Semiconductors, ed. B.L.H. Wilson (Conf. Series 1/43, Inst. Phys., London, 19791, p. 1305.
